Electroplated Copper Interfacial Micro Void Nucleation Control
Controlling copper interfacial micro-void nucleation requires balancing plating bath organic additives, limiting total organic carbon, and enforcing slow thermal anneal ramps.
Interfacial Micro Voids
Meaning
Small cavities forming at the boundary between two distinct material layers represent a significant reliability risk in semiconductor packaging and printed circuit board assembly. These interfacial micro voids often appear at the junction between a copper pad and the solder alloy, particularly when using lead free finishes. They weaken the mechanical integrity of the joint and can lead to electrical failure under thermal stress.
Formation Mechanism
The migration of atoms during high temperature processes drives the growth of these defects. When tin reacts with copper, it forms intermetallic compounds that can leave behind interfacial micro voids due to the Kirkendall effect. This occurs when one species of metal diffuses faster than the other, resulting in a vacancy imbalance at the interface.
Surface Influence
The quality of the plating on the base metal significantly affects the density of the cavities. Electroless nickel immersion gold finishes are susceptible to a specific type of interfacial micro voids known as black pad. Contamination in the plating bath or excessive gold thickness can trigger the corrosion of the underlying nickel during the soldering process.
Detection Methods
High resolution cross sectional analysis and scanning electron microscopy are required to identify these microscopic gaps. Because interfacial micro voids are often too small for standard x-ray inspection, they remain hidden until a mechanical failure occurs. Regular pull testing and thermal cycling provide data on the long term stability of the bonded surfaces.